Filter circuit and filter integrated circuit

ABSTRACT

A filter circuit and a filter integrated circuit capable of being used in a high frequency band comprises a first resistor R 1  connected between an input signal source and an emitter of a common-base transistor TR 1 , a first capacitor C 1  connected between said input signal source and a reference voltage point, a second capacitor C 2  connected between said input signal source and a collector of the common-base transistor TR 1 , and a second resistor R 2  connected between the collector of the common-base transistor and the reference voltage point. Thus, a low-pass filter which operates in a high frequency band and suppresses the influence of characteristic parameters over the filter characteristic can be constructed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a filter circuit using a transistor, and a filter integrated circuit formed on an integrated circuit.

2. Description of the Related Art

In an optical communication system or a radio communication system using high speed signals of several gigabits per seconds [Gb/s] or higher, high-pass, low-pass, and band-pass filters which operate in a high frequency band of several gigahertz [GHz] or higher are sometimes required. For example, in a timing circuit limiter amplifier for a 3R type optical repeater, a band-pass amplifier or the like using a tuning circuit and so forth is employed so as to suppress jitter caused by circuit noise and to prevent an output waveform from being linked by harmonics.

Generally, a filter circuit requires inductance as well as resistors and capacitors. However, it is difficult to form inductance in an integrated circuit (IC) device. Thus, a filter circuit is constructed by connecting an external coil. Alternatively, the entire filter circuit is used as an external part.

However, when inductance is provided as an external part, as the frequency increases, stray capacitance, stray inductance, and so forth increase. As a result, the characteristic of the filter in a high frequency band thereof degrades.

To solve this problem, a filter is constructed of an operational (OP) amplifier, capacitors, and resistors which can be used for an IC device. This type of filter is referred to as an active filter.

FIGS. 1A and 1B are schematic diagrams showing bridge T type circuits. FIG. 2 is a schematic diagram showing a circuit of a band-pass filter which is constructed of a bridge T type circuit and an OP amplifier.

FIG. 3 is a schematic diagram showing a circuit of an active high-pass filter using an OP amplifier. FIG. 4 is a schematic diagram showing a circuit of an active low-pass filter referred to as a Sallen-Key circuit.

In the above-mentioned active filter using an OP amplifier, since the characteristic of the filter depends on the frequency characteristic of the OP amplifier, the filter can operate at up to several megahertz [MHz] rather than in a high frequency band of several gigahertz [GHz] or higher.

To solve this problem, a conventional wide-band amplifier instead of the OP amplifier can be used. However, even with a wide-band amplifier, since the phase characteristic in the high frequency band varies, a desired filter characteristic may not be obtained or an oscillation may take place.

As shown in FIGS. 3 and 4, an active filter using a voltage follower which is constructed of an emitter follower circuit of a transistor can provide a wide-band filter although it cannot be applied to the band-pass filter using a negative feedback circuit of an OP amplifier as shown in FIG. 2.

FIGS. 5 and 6 are schematic diagrams showing circuits of a high-pass filter and a low-pass filter, each of which uses an emitter follower. In these filters, the OP amplifiers shown in FIGS. 3 and 4 are substituted with emitter follower circuits which are constructed of transistors.

However, in the Sallen-Key low-pass filter using the emitter follower shown in FIG. 6, when the filter operates in a high frequency band of gigahertz [GHz] or higher, a designed filter characteristic cannot be obtained due to the influence of the capacitance and resistance of the transistor.

Next, the influence of resistance, capacitance, and so forth of a transistor over a Sallen-Key low-pass filter using an emitter follower circuit shown in FIG. 6 will be described.

FIG. 7 is a schematic diagram showing an equivalent circuit, when operating in a high frequency band, of the Sallen-Key low-pass filter using the emitter follower circuit shown in FIG. 6. As shown in the FIG. 7, a base resistance R_(b), an emitter resistance R_(e), a base-collector capacitance C_(bc), and so forth of the transistor are added to the low-pass filter shown in FIG. 6. Thus, in the circuit shown in FIG. 7, the low-pass filter which consists of R₁, R₂, C₁, and C₂ is connected to the low-pass filter which consists of R_(b) and C_(bc) in series. As a result, when the frequency band of the filter which consists of R₁, R₂, C₁, and C₂ is much lower than that of the filter which consists of R_(b) and C_(bc), a desired frequency characteristic can be obtained. However, when a low-pass filter which operates in a much higher frequency band is required, the frequency characteristic of the filter which consists of R_(b) and C_(bc) cannot be ignored.

Moreover, in a high frequency band, since the emitter resistance R_(e) influences the output impedance of the emitter follower circuit, the low impedance intrinsic to the emitter follower circuit cannot be obtained.

FIG. 8A and 8B are schematic diagrams and graphs showing equivalent circuits and gains in a high frequency band (in the vicinity of the cutoff frequency) of a Sallen-Key low-pass filter. FIG. 8A shows an equivalent circuit and a gain in the ideal case where the filter is not affected by the parameters of the transistor. FIG. 8B shows an equivalent circuit and a gain in the case where the filter is affected by the emitter resistance R_(e) of the transistor. In the equivalent circuits of FIG. 8, in the high frequency band, it is assumed that the capacitor of the filter is shorted.

At the cutoff frequency or high of the filter, the ideal low-pass filter shown in FIG. 8A has a particular attenuating characteristic. On the other hand, in the case where the influence of the emitter resistor R_(e) of the transistor cannot be ignored as shown in FIG. 8B, at the cutoff frequency or higher of the filter, the gain of the filter is larger than a particular value. Thus, in this case, the filter does not have the characteristic of a low-pass filter.

FIG. 9 is a graph showing the frequency characteristic of the conventional low-pass filter using the emitter follower of FIG. 6. The solid line of the graph represents calculated values of an ideal characteristic. The dot line of the graph represents a simulated result using real transistor parameters.

As shown in FIG. 9, at around six gigahertz [6GHz] before which is lower than the cutoff frequency, the output of the filter temporarily drops due to the influence of the base-collector capacitance C_(bc) and the base resistance R_(b) of the transistor. Thereafter, the stopping characteristic of the filter in the cutoff region degrades due to the influence of the emitter resistance R_(e) of the transistor. As a result, a low-pass filter using an emitter follower cannot be used in a high frequency band of several gigahertz [GHz] or higher.

In addition, a circuit has been necessarily integrated in considerations of cost and reliability. In particular, the shorter the wavelength of a signal in a high frequency band, the more the analysis of a distributed constant of the circuit becomes important. Thus, the filter should be formed on an IC device.

When a filter is formed on an IC device, the frequency characteristic of the filter should be variably controlled so as to change the frequency band thereof and to compensate for the characteristic deviation which takes place in the production stage from device to device.

As a related art which can solve the above-mentioned problem, for example Japanese Patent Laid-Open Serial No. 1984-215111 is known, which discloses a technology of adjusting a filter characteristic by changing a resistance of a diode after replacing a resistor used in a Sallen-Key type filter by the diode so as to adjust the operating characteristic of the filter.

However, it is likely that the object of the above-mentioned related art is to adjust the characteristic of the Sallen-Key low-pass filter rather than adjust the frequency characteristic of a filter which can operate in a high frequency band of several gigahertz [GHz] or higher.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a filter circuit and a filter integrated circuit. Another object of the present invention is to provide a filter circuit and a filter integrated circuit which can control a frequency characteristic in a high frequency band thereof.

The first aspect of the present invention is a first resistor R₁ connected between an input signal source and an emitter of a common-base transistor TR₁, a first capacitor C₁ connected between the input signal source and a reference voltage point, a second capacitor C₂ connected between the input signal source and a collector of the common-base transistor TR₁, and a second resistor R₂ connected between the collector of the common-base transistor and the reference voltage point.

The second aspect of the present invention is a first capacitor C₃ and a second capacitor C₄ connected in series between an input signal source and a base of an emitter follower transistor TR₅, a first diode D₃ connected between the base of the emitter follower transistor TR₅ and a reference voltage point, a second diode D₄ connected between an emitter of the emitter follower transistor TR₅ and the connection point of the first and second capacitors C₃ and C₄, a first variable current source I₂ connected between a cathode of the first diode and the reference voltage point, and a second variable current source I₃ connected between a cathode of the second diode and the reference voltage point.

The current transfer characteristic of the input and output of the above-mentioned low-pass filter can be given by the following formula.

    I.sub.out /I.sub.in =(1+sR.sub.1 C.sub.1)/[s.sup.2 C.sub.1 R.sub.1 C.sub.2 R.sub.2 +sR.sub.1 (C.sub.1 +C.sub.2)+1]

At this point, the polar angular frequency ω is given by the following formula.

    ω=(C.sub.1 R.sub.1 C.sub.2 R.sub.2).sup.-1/2

Thus, when the relation of R₂ >R₁ and C₁ ≈C₂ are satisfied, nearly at the pole, the numerator of the above formula can be treated as "1". As a result, the formula represents the characteristic of a low-pass filter.

When a low-pass filter is constructed using a common-base transistor, the variation of the input voltage of the transistor can be decreased and the influence of the capacitance over the transistor can be suppressed. As a result, a filter which can satisfactorily operate in a high frequency band can be constructed.

Moreover, in the above-mentioned high-pass filter, by varying the output current of the variable current sources I₁ and I₂, the resistance of the diodes D₃ and D₄ can be controlled. As a result, the frequency characteristic of the filter can be freely adjusted.

Thus, when the filter is formed in an IC, the frequency band can be varied depending on its application and the characteristic deviation which is caused in the production stage of the IC from device to device can be compensated for.

These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic diagrams showing conventional bridge T type circuits;

FIG. 2 is a schematic diagram showing a circuit of a conventional band-pass filter using an OP amplifier;

FIG. 3 is a schematic diagram showing a circuit of a conventional active high-pass filter;

FIG. 4 is a schematic diagram showing a circuit of a conventional active low-pass filter;

FIG. 5 is a schematic diagram showing a circuit of a conventional active high-pass filter using an emitter follower;

FIG. 6 is a schematic diagram showing a circuit of conventional active low-pass filter using an emitter follower;

FIG. 7 is a schematic diagram showing an equivalent circuit of a conventional low-pass filter;

FIGS. 8A and 8B are schematic diagrams and graphs describing conventional low-pass filters;

FIG. 9 is graph showing a frequency characteristic of a conventional low-pass filter;

FIG. 10 is a schematic diagram showing a circuit of a low-pass filter of a first embodiment in accordance with the present invention;

FIG. 11 is a schematic diagram showing an equivalent circuit of the low-pass filter of the first embodiment;

FIG. 12 is a schematic diagram showing a circuit of a low-pass filter of a second embodiment;

FIG. 13 is a schematic diagram showing a circuit of a low-pass filter of a third embodiment;

FIG. 14 is a graph showing the frequency characteristic of the low-pass filter of the third embodiment;

FIG. 15 is a circuit diagram showing a circuit of a band-pass filter of a forth embodiment;

FIG. 16 is a circuit diagram showing a circuit of a high-pass filter of a fifth embodiment;

FIG. 17 is a graph showing a frequency characteristic of the high-pass filter of the fifth embodiment;

FIG. 18 is a graph showing a frequency characteristic of a band-pass filter with a low-pass side control; and

FIG. 19 is a graph showing a frequency characteristic of the band-pass filter with a high-pass side control.

DESCRIPTION OF PREFERRED EMBODIMENTS

Next, with reference to the accompanying drawings, embodiments in accordance with the present invention will be described. FIG. 10 is a schematic diagram showing a circuit of an active low-pass filter of a first embodiment in accordance with the present invention.

In FIG. 10, a resistor R₁ is connected between an input signal source and the emitter of a common-base transistor TR₁. A capacitor C₁ is connected between the input signal source and a ground plane (reference voltage point). A capacitor C₂ is connected between the collector of the transistor TR₁ and the input signal source. A resistor R₂ is connected between the collector of the transistor TR₁ and the ground surface.

In this circuit, a signal current which is received from the signal source is denoted by I_(in) ; an output current which flows in the register R₂ by I_(out), a voltage of the capacitor C₁ by V_(in) ; and an emitter current which flows in the register R₁ by I_(e).

Since the emitter current I_(e) of the common-base transistor TR₁ is nearly equal to the collector current, a current I_(c2) which flows in the capacitor C₂ can be expressed by the following formulas.

    I.sub.c2 =I.sub.out -I.sub.e and

    I.sub.c2 =I.sub.in -(I.sub.e +jωC.sub.1 V.sub.in)

    Thus,

    I.sub.out -I.sub.e =I.sub.in -(I.sub.e +jωC.sub.1 V.sub.in)

Therefore, the following formula can be obtained.

    V.sub.in =(I.sub.in -I.sub.out)/jωC.sub.1            (1)

Since the voltage V_(in) of the capacitor C₁ is given by the following formula

    V.sub.in =(I.sub.out -I.sub.e)/jωC.sub.2 +R.sub.2 I.sub.out

    Thus,

    I.sub.out -I.sub.e =(V.sub.in -R.sub.2 I.sub.out)jωC.sub.2

By substituting I_(e) =V_(in) /R₁ into the above formula, the following formula can be obtained.

    I.sub.out -V.sub.in /R.sub.1 =(V.sub.in -R.sub.2 I.sub.out)jωC.sub.2( 2)

Substituting the formula (1) into the formula (2)

    I.sub.out -(I.sub.in -I.sub.out)/jωC.sub.1 R.sub.1 =[(I.sub.in -I.sub.out)/jωC.sub.1 -R.sub.2 I.sub.out ]jωC.sub.2

By arranging the above formula, the following formula can be obtained.

    jωC.sub.1 R.sub.1 I.sub.out -(I.sub.in -I.sub.out)=jωC.sub.2 R.sub.1 (I.sub.in -I.sub.out)-jωC.sub.1 R.sub.1 jωC.sub.2 R.sub.2 I.sub.out

In addition,

    (jωC.sub.1 R.sub.1 jωC.sub.2 R.sub.2 +jωC.sub.1 R.sub.1 +jωC.sub.2 R.sub.1 +1)I.sub.out =(1+jωC.sub.2 R.sub.1)I.sub.in

Thus, the current transfer characteristic of the input and output can be given by the following formula.

    I.sub.out /I.sub.in =(1+jωC.sub.2 R.sub.1)/[(jω).sup.2 C.sub.1 R.sub.1 C.sub.2 R.sub.2 +jωR.sub.1 (C.sub.1 +C.sub.2)+1](3)

Moreover, the above formula can be expressed with a complex angular frequency s.

    I.sub.out /I.sub.in =(1+sR.sub.1 C.sub.2)/[s.sup.2 C.sub.1 R.sub.1 C.sub.2 R.sub.2 +sR.sub.1 (C.sub.1 +C.sub.2)+1]                   (4)

At this point, the polar angular frequency and Q of the filter can be given by the following formulas.

    ω=(C.sub.1 R.sub.1 C.sub.2 R.sub.2).sup.-1/2

    Q=(R.sub.2 /R.sub.1).sup.1/2 ·(C.sub.1 C.sub.2).sup.1/2 /(C.sub.1 +C.sub.2)

Although the formula (4) is precisely not a transfer characteristic formula of a low-pass filter, when the relations of R₂ >R₁ and C₁ ≈C₂ are satisfied so as to increase the value of Q, sR₁ C₂ of the numerator of the formula (4) nearly at the polar frequency can be transformed into R₁ C₂ / (C₁ R₁ C₂ R₂)^(1/2). Thus, when the relations of R₂ =nR₁ and C₁ ≈C₂ are satisfied, the relation of sR₁ C₂ ≈(n)^(-1/2) is obtained. Thus, the numerator of the formula (4) can be treated as nearly "1".

In other words, the formula (4) represents the transfer characteristic of a low-pass filter. Thus, a low-pass filter can be constructed using a common-base transistor.

FIG. 11 shows an equivalent circuit, when operating in a high frequency band, of a low-pass filter using a common-base transistor. In this equivalent circuit, the emitter resistance R_(e) of the transistor is connected to the resistor R₁ of the filter in series. Thus, when a particular value of the resistor R₁ is selected, a desired characteristic of the filter can be obtained. In the common-base transistor, the base resistance R_(b) and the base-collector capacitance C_(bc) are connected to the resistor R₂ in parallel. Thus, when the value of the resistor R₂ is large, the impedance formed by the base-resistance R_(b) and the base-collector capacitance C_(bc) affects the characteristic of the filter. However, when a low-pass filter is used in a high frequency band, since a small value is assigned to the resistor R₂, it is possible to prevent the base resistance R_(b) and the base-collector capacitance C_(bc) from affecting the characteristic of the filter.

Here, exemplary values of C₁, C₂, R₁ and R₂ are shown. The values of capacitors C₁ and C₂ are must be more than a minimum to shut an effect of the internal capacitance of a transistor. If the capacitors C₁ and C₂ have capacitance values approximately at zero point five picofarads [0.5 pF], the values of resistors R₁ and R₂ of a low-pass filter having a cutoff frequency of ten gigahertz [10 GHz] are about ten ohms [10Ω] and twenty ohms [20Ω], for example. That is, R₁ ≈10Ω and R₂ ≈20Ω, when C₁ =C₂ =0.5 pF.

When the value of R₂ is like the above, the composite impedance of the transistor due to the base-collector capacitance C_(bc) and the base resistance R_(b) has little effect in a high frequency band over a few gigahertz [GHz].

In other words, when a low-pass filter is constructed by using a common-base transistor, the filter can operate in a high frequency band of several gigahertz [GHz] or higher and suppress the influence of the inner resistance and capacitance thereof over the frequency characteristic.

Next, with reference to FIG. 12, a low-pass filter of a second embodiment in accordance with the present invention will be described. In the low-pass filter shown in FIG. 12, an emitter follower circuit is connected to an output of the common-base transistor shown in FIG. 10 so as to obtain an output signal as a voltage signal.

In the circuit shown in FIG. 12, a resistor R₅ is connected between the collector of a common-base transistor TR₁ and a ground surface. The collector of the common-base transistor TR₁ is connected to the base of an emitter follower transistor TR₂.

The transfer characteristic of the input and output of this filter circuit can be given by the following formula.

    V.sub.out /I.sub.IN =R.sub.5 /[s.sup.2 C.sub.1 R.sub.1 C.sub.2 R.sub.2 +s(C.sub.1 R.sub.1 +C.sub.2 R.sub.1 +C.sub.2 R.sub.2 -C.sub.2 R.sub.5)+1]

In addition, the polar angular frequency ω and Ω of the filter of this circuit can be given by the following formulas.

    ω=(C.sub.1 R.sub.1 C.sub.2 R.sub.2).sup.-1/2

    Q=(C.sub.1 R.sub.1 C.sub.2 R.sub.2).sup.1/2 /(C.sub.1 R.sub.1 +C.sub.2 R.sub.1 +C.sub.2 R.sub.2 -C.sub.2 R.sub.5)

When the values of C₂ and R₅ are set in such a way that the relation of C₂ R₅ =C₁ R₁ +C₂ R₁ C₂ R₂ is satisfied, the transfer characteristic of the filter can be given by the following formula.

    V.sub.out /I.sub.IN =R.sub.5 /[s.sup.2 C.sub.1 R.sub.1 C.sub.2 R.sub.2 +1]

This formula represents the transfer characteristic of the filter with a resonant frequency of ω=1/(C₁ R₁ C₂ R₂)^(-1/2). Thus, when the above-mentioned value of C₂ R₅ is set, the filter of FIG. 12 can be used as a band-pass filter.

Next, with reference to FIG. 13, an active low-pass filter of a third embodiment in accordance with the present invention will be described. The active low-pass filter shown in FIG. 13 is constructed by using common-base transistors. In this active low-pass filter, the frequency characteristic can be controlled.

In the circuit shown in FIG. 13, an emitter resistance R_(e) of a first common-base transistor is used instead of the resistor R₁ of FIG. 10. In addition, an emitter resistance of a second common-base transistor and the resistance of a diode D₁ which are connected in series are used instead of the resistance R₂ of FIG. 10. The anode of the diode D₁ is connected to the emitter of the second common-base transistor TR₃. From the collector of the second common-base transistor, a signal current is obtained.

In addition, a variable current source I₁ which is used to control the values of the resistance of the emitter resistance R_(e) and the diode D₁ is connected between the emitter of the common-base transistor TR₁ and the ground surface.

In this low-pass filter, by varying the output current of the variable current source I₁, the values of the resistance of the emitter resistance R_(e) and the diode D₁ can be varied so as to control the cutoff frequency. As a result, since the cutoff frequency of the filter can be varied in a predetermined range, a desired frequency characteristic thereof can be obtained. In addition, the characteristic deviation of the filter which is caused in the production stage of the IC from device to device can be compensated.

FIG. 14 is a graph showing an example of the frequency characteristic of the above-mentioned low-pass filter. In the graph, the output of the filter slightly increases due to the influence of the characteristic of the transistor in the vicinity of the cutoff frequency of ten gigahertz [10 GHz]. However, in a high frequency band of several gigahertz [GHz] or higher, a nearly desired frequency characteristic can be obtained.

The four dot lines of the graph represent cutoff characteristics of the filter where the output current of the variable current source I₁ is varied. Thus, this graph shows that by varying the output current of the variable current source I₁ the cutoff frequency of the filter can be controlled.

Next, with reference to FIG. 15, a band-pass filter of a fourth embodiment in accordance with the present invention will be described. In the band-pass filter shown in the figure, a high-pass filter HP using an emitter follower circuit and the low-pass filter LP using a common-base transistor shown in FIG. 13 are combined.

The construction of the high-pass filter HP of FIG. 15 is basically the same as that of the conventional high-pass filter using the emitter follower circuit of FIG. 5. On the other hand, the construction of the low-pass filter LP is basically the same as that of the low-pass filter of FIG. 13. However, in the band-pass filter shown in FIG. 15, resistors R₁ and R₂ are used instead of the emitter resistance Re and the diode D₁ of the low-pass filter of FIG. 13. Thus, the cutoff frequency of the band-pass filter is fixed.

The output signal voltage of the high-pass filter HP of FIG. 15 is input to the base of an emitter follower transistor TR₆. The impedance of both the HP circuit and LP circuit is converted by the transistor TR₆. The resultant signal is input to the base of a transistor TR₇ which constructs a differential amplifying circuit. The collector of the transistor TR₇ is grounded. The emitter of the transistor TR₇ is connected to a current source which is common with the emitter of the transistor TR₈.

The collector of the transistor TR₈ of the differential amplifying circuit is connected to an input of the low-pass filter LP. A resistor R₆ is connected between the collector of the transistor TR₃ and the ground surface. The collector of the transistor TR₃ is an output of the low-pass filter.

In this band-pass filter, frequency components of an input signal V_(in) which are power than the cutoff frequency of the high-pass filter HP are cut. A voltage signal which passes through the high-pass filter HP is sent to the differential amplifying circuit constructed of the transistors TR₇ and TR₈ through the emitter follower circuit constructed of the transistor TR₆. The differential amplifying circuit converts the voltage signal into a current signal. The resultant current signal is sent to the low-pass filter LP constructed of a common-base transistor. The low-pass filter cuts frequency components higher than the cutoff frequency. Thus, the resultant signal is output from an output resistor R₆ as an output signal V_(out).

With a low-pass filter using a common-base transistor, a band-pass filter which operates in a high frequency band of several gigahertz [GHz] or higher and suppresses the influence of the inner resistance and capacitance of the transistor can be constructed.

In addition, a low-pass filter can be constructed by using first and second diodes instead of the resistors R₁ and R₂ of the low-pass filter LP of the band-pass filter of FIG. 15 and connecting a variable current source between the input signal source and the reference voltage point. In this case, by varying the output current of the variable current source, the values of the resistance of the first and second diodes can be controlled so as to adjust the cutoff frequency of the low-pass filter. As a result, a filter which has a desired frequency characteristic and which can compensate for the deviation of the frequency characteristic which takes place in the IC device production stage can be constructed.

It should be noted that the circuit which converts a voltage signal of the high-pass filter into a current signal is not limited to a differential amplifying circuit, but other circuits may be used. In addition, the present invention can be applied to a band-stop filter which stops frequency components in a predetermined frequency band.

Next, with reference to FIG. 16, a high-pass filter of a fifth embodiment in accordance with the present invention will be described. In the high-pass filter shown in the figure, an emitter follower circuit is used so as to control the cutoff frequency.

In the high-pass filter shown in FIG. 16, diodes D₃ and D₄ are used instead of the resistors R₃ and R₄ of the conventional high-pass filter shown in FIG. 5. A variable DC current source I₂ is connected to the cathode of the diode D₃. In addition, a variable DC current source I₃ is connected to the cathode of the diode D₄. In this high-pass filter, by varying the output currents of the variable DC current sources I₂ and I₃, the values of the resistance of the diodes D₃ and D₄ are varied so as to freely control the cutoff frequency of the filter.

FIG. 17 is a graph showing an example of measured values of a frequency characteristic of the above-mentioned high-pass filter. The four dot lines of the graph represent cutoff characteristics of the filter when the output currents of the variable DC current sources I₂ and I₃ are varied. Thus, by varying the output currents of the variable DC current sources I₂ and I₃, the cutoff frequency can be freely controlled.

In addition, a band-pass filter can be also constructed by combining the high-pass filter of the fifth embodiment which can variably control the cutoff frequency and the low-pass filter of the third embodiment which can also variably control the cutoff frequency or a low-pass filter with a fixed cutoff frequency where the diode of the third embodiment is replaced with a resistor.

FIGS. 18 and 19 are graphs showing frequency characteristics of a band-pass filter where the high-pass filter of the fifth embodiment which can variably control the cutoff frequency and the low-pass filter of the third embodiment which can variably control the cutoff frequency are combined.

FIG. 18 is a graph showing simulated values of the frequency characteristic of the above-mentioned band-pass filter with the low-pass side control. The four dot lines of the graph represent cutoff characteristics of the filter when the output current of the DC current source is varied.

Likewise, FIG. 19 is a graph showing simulated values of the frequency characteristic of the above-mentioned band-pass filter with the high-pass side control. The four dot lines of the graph represent the cutoff characteristics of the filter when the output current of the DC current source is varied.

When a band-pass filter is constructed of the high-pass filter and the low-pass filter which can both control the cutoff frequency, the band-pass filter can operate in a high frequency band and freely control the high-band and low-band cutoff frequencies.

According to the present invention, when a filter is constructed by using a common-base transistor, a low-pass filter, a band-pass filter, and so forth which operate in a high frequency band of several gigahertz [GHz] or higher with suppression of the influence of inner stray capacitance of the transistor can be constructed. In addition, by using a diode instead of a resistor in a high-pass filter using an emitter follower circuit, the current which flows through the diode can be varied and thereby the frequency characteristic of the high-pass filter can be freely controlled.

Although the present invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing form the spirit and scope of the present invention. 

What is claimed is:
 1. A low-pass filter circuit, comprising:a common-base transistor; a first resistor connected between an input signal source and an emitter of a common-base transistor; the first capacitor connected between said input signal source and a reference voltage point; a second capacitor connected between said input signal source and a collector of the common-base transistor; and a second resistor connected between the collector of the common-base transistor and the reference voltage point.
 2. The low-pass filter circuit as set forth in claim 1, wherein:the resistance value of said second resistor is set to be relatively small and thus the internal impedance of said common-base transistor connected in parallel with said second resistor is relatively large enough to be negligible in a high frequency band.
 3. The low-pass filter circuit as set forth in claim 1, wherein:the resistance value of said second resistor is set to be relatively small and thus the composite impedance of said common-base transistor, connected in parallel with said second resistance, due to its base resistance and base-collector capacitance is large enough to be negligible in a high frequency band.
 4. A low-pass filter integrated circuit, comprising:a common-base transistor; a first resistor connected between an input signal source and an emitter of said common-base transistor; a first capacitor connected between said input signal source and a reference voltage point; a second capacitor connected between said input signal source and a collector of the common-base transistor; and a second resistor connected between the collector of the common-base transistor and the reference voltage point. 